Port --enable-gcc-warnings to clang.

Port --enable-gcc-warnings to clang.

* configure.ac (nw): Remove obsolescent warnings.
These aren't needed for clang, or for gcc for that matter.
(emacs_cv_clang): New var, which tests for clang.
Omit warnings that clang is too picky about.
(GLIB_DISABLE_DEPRECATION_WARNINGS): Define this;
needed for Ubuntu 13.04 + clang + --enable-gcc-warnings.
* lib-src/etags.c: Omit unnecessary forward decls.
(print_version, print_help): Declare _Noreturn.
* lib-src/pop.c (socket_connection) [HAVE_GETADDRINFO]: Simplify.
* src/bytecode.c (exec_byte_code):
* src/regex.c:
Redo diagnostic pragmas to pacify clang, too.
* src/dbusbind.c (xd_retrieve_arg): Do not use uninitialized variable.
* src/editfns.c (Fencode_time):
* src/fileio.c (file_accessible_directory_p):
* src/font.c (font_unparse_xlfd):
Use '&"string"[index]' instead of '"string" + (index)'.
* src/undo.c (user_error): Remove; unused.